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The textbook Japanese for busy people 1 tells me, in lesson 6, that "relative time expressions like あした, らいしゅう, こんげつ and きょねん generally do not take any particles."
They exemplify this with sentences like:

スミスさんはあしたぎんこうにいきます。
チャンさんはきょねんにほんにきました。

One lesson later, they tell me that "unlike relative time expressions, specific time expressions take the particle に.

どようびに, 'on Saturday'
２００６ねんに, 'in 2006'"

They then provide these example sentences:

ジョンソンさんはらいしゅうのきんようびににほんにきます。
かいぎはすいようびです。

However, and contradicting the explanation given, the second example sentence does not take に. Furthermore, they do not provide any example as to why. Is there any rule that explains this? When do specific time expression take the に particle, and when don't they?

Depends how you count, but when you consider だ and で as different forms of です, then it only happens with です (and its two other incarnations). In other words, there is only one copula in Japanese, which inflects to です, だ and で. Each of these may act as copula, and if it does, then a time reference, like すいようび, is not an adverb and doesn't take に.
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EarthliŋMar 18 '13 at 16:41